U.S. patent number 9,616,415 [Application Number 14/007,167] was granted by the patent office on 2017-04-11 for steam re-calcination of mixed metal oxide catalysts.
This patent grant is currently assigned to ROHM AND HAAS COMPANY. The grantee listed for this patent is Jinsuo Xu. Invention is credited to Jinsuo Xu.
United States Patent |
9,616,415 |
Xu |
April 11, 2017 |
Steam re-calcination of mixed metal oxide catalysts
Abstract
A process for producing a catalyst for the (amm)oxidation of
alkanes comprises calcination of a crystalline mixed metal oxide
catalyst partially or wholly in the presence of steam.
Inventors: |
Xu; Jinsuo (Fort Washington,
PA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Xu; Jinsuo |
Fort Washington |
PA |
US |
|
|
Assignee: |
ROHM AND HAAS COMPANY
(Philadelphia, PA)
|
Family
ID: |
45922838 |
Appl.
No.: |
14/007,167 |
Filed: |
March 21, 2012 |
PCT
Filed: |
March 21, 2012 |
PCT No.: |
PCT/US2012/029857 |
371(c)(1),(2),(4) Date: |
September 24, 2013 |
PCT
Pub. No.: |
WO2012/134898 |
PCT
Pub. Date: |
October 04, 2012 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140031585 A1 |
Jan 30, 2014 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
61467418 |
Mar 25, 2011 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J
37/0036 (20130101); B01J 27/0576 (20130101); B01J
37/10 (20130101); C07C 51/215 (20130101); B01J
35/002 (20130101); B01J 23/002 (20130101); B01J
37/036 (20130101); B01J 23/6525 (20130101); C07C
51/215 (20130101); C07C 57/04 (20130101); B01J
2523/00 (20130101); B01J 2523/00 (20130101); B01J
2523/55 (20130101); B01J 2523/56 (20130101); B01J
2523/64 (20130101); B01J 2523/68 (20130101); B01J
2523/824 (20130101) |
Current International
Class: |
C07C
51/215 (20060101); B01J 23/00 (20060101); B01J
27/057 (20060101); B01J 23/652 (20060101); B01J
35/00 (20060101); B01J 37/00 (20060101); B01J
37/03 (20060101); B01J 37/10 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1260495 |
|
Aug 2004 |
|
EP |
|
2009106474 |
|
Sep 2009 |
|
WO |
|
Other References
X-ray Diffraction (XRD) and Reflectivity (XRR)., MRL Frederick
Seitz Materials Research Laboratory.,
http://mrl.illinois.edu/facilities/center-microanalysis-materials/cmm-ins-
truments/x-ray-diffraction-xrd-and-reflectivity-xrr. 2014. cited by
applicant .
X-ray Diffraction., The University of Manchester School of
Materials.,
http://www.materials.manchester.ac.uk/our-research/facilities/x-ray-diffr-
action/. cited by applicant .
X-Ray Facility Instruments. Materials Research Laboratory at UCSB:
An NSF MRSEC. www.mrl.uscb.edu/centralfacilities/x-ray/instruments.
cited by applicant .
X-ray Diffraction Tubes: Seifert Analytical X-ray. GE Measurement
and Control Solutions. www.ge-mcs.com. 2011. cited by applicant
.
Holmberg, et al., "A study of propane ammoxidation on
Mo--V--Nb--Te-oxide catalysts diluted with A12O3, SiO2, and TiO2",
Journal of Catalysis, 243, 2006, p. 350-359. cited by applicant
.
Florea, et al., "High surface area Mo--V--Te--Nb--O catalysts:
Preparation, characterization and catalytic behaviour in
ammoxidation of propane", Catalysis Today, 112, 2006, p. 139-142.
cited by applicant .
Ueda, et al, "Crystalline Mo--V--O based complex oxides as
selective oxidation catalysts of propane", Catalysis Today, 99,
2005, p. 43-49. cited by applicant .
Wagner, et al., "Surface texturing of Mo--V--Te--Nb--Ox selective
oxidation catalysts", Topics in Catalysis, vol. 38, Nos. 1-3, Jul.
2006, p. 51-58. cited by applicant .
Feng, et al., "The study on the source of Te and the dispersion of
TeO2 in fabricating Mo--V--Te and Mo--V--Te--Nb mixed metal oxide
catalysts for propane partial oxidation", Journal of Molecular
Catalysis A: Chemical, 267, 2007, p. 245-254. cited by
applicant.
|
Primary Examiner: Li; Jun
Attorney, Agent or Firm: Crimaldi; Kenneth
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from provisional application Ser.
No. 61/467,418, filed Mar. 25, 2011, which is incorporated herein
by reference in its entirety.
Claims
What is claimed is:
1. A process, comprising the steps of: (a) providing a mixed metal
oxide first catalyst having the empirical formula:
MoV.sub.aNb.sub.bX.sub.cZ.sub.dO.sub.n and an X-ray diffraction
pattern showing the orthorhombic phase as the major crystal phase
with main peaks with 2.theta. at 6.7.degree., 7.8.degree.,
22.1.degree., and 27.2.degree., wherein X is Te, Z is Pd, a=0.2 to
0.3, b=0.1 to 0.3, c=0.15 to 0.3, d=0.005 to 0.02, and n is
determined by the oxidation states of the other elements; wherein a
catalyst precursor is dried and then calcined at a temperature from
550 to 650.degree. C. in an inert atmosphere consisting of
nitrogen, argon, xenon, helium or mixtures thereof to produce the
first catalyst; and (b) calcining the first catalyst at ambient
atmospheric pressure at a temperature from 100 to 550.degree. C.
partly or wholly in an atmosphere that comprises water vapor to
form a mixed metal oxide final catalyst.
2. The process of claim 1 further comprising: (c) grinding the
mixed metal oxide final catalyst.
3. The process of claim 1 wherein the water vapor comprises from
0.01 to 100 volume percent, based on the total volume of gas in the
atmosphere.
4. The process of claim 1 wherein the water vapor comprises from
0.5 to 10 volume percent, based on the total volume of gas in the
atmosphere.
5. The process of claim 1 wherein the water vapor comprises from 1
to 3.5 volume percent, based on the total volume of gas in the
atmosphere.
6. The process of claim 1 wherein calcining the catalyst precursor
is done in 2 stages, wherein, in the first stage, the catalyst
precursor is calcined in an inert or oxidizing environment at a
temperature of from 200 to 330.degree. C., for from 15 minutes to
40 hours, and in a second stage the material from the first stage
is calcined in an inert atmosphere at a temperature of from 550 to
650.degree. C. for 0.1 to 25 hours.
7. The process of claim 1 wherein the first catalyst has the
empirical formula: MoV.sub.aNb.sub.bX.sub.cZ.sub.dO.sub.n wherein a
is 0.285, b is 0.164, c is 0.21, and d is 0.01.
Description
BACKGROUND OF THE INVENTION
The present invention relates to mixed metal oxide (amm)oxidation
catalysts.
Mixed metal oxide (MMO) catalysts are well known for (amm)oxidation
of alkanes and alkenes to unsaturated carboxylic acids or nitrites.
For example, U.S. Pat. No. 6,407,280 discloses MMO catalysts
containing Mo or W; V or Ce; Te, Sb or Se; as well as other metals,
e.g., Nb, and promoted by at least one of Ni, Pd, Cu, Ag and
Au.
U.S. Pat. No. 7,875,571 discloses a method for preparing MMO
catalysts by contacting an MMO with water alone at elevated
temperature or water comprising a metal oxide precursor to form a
modified MMO, followed by calcining the modified MMO.
U.S. Pat. No. 7,304,014 teaches preparing MMO catalysts by, after
calcination, doing one or more chemical and/or physical treatments
such as extraction with oxalic acid in methanol, extraction with an
alcohol followed by densification by extra pressing, cryo-grinding
followed by extraction with oxalic acid in methanol, and
others.
Prior art processes for preparing selective MMO catalysts are
complicated and can involve a large number of steps. Each
additional step incurs additional costs, and can lead to catalysts
that give relatively more variation in selectivity during use. It
would be desirable to have a relatively simple method of preparing
selective MMO catalysts.
SUMMARY OF THE INVENTION
The present disclosure provides such a process, comprising the
steps of:
(a) providing a mixed metal oxide first catalyst having the
empirical formula: MoV.sub.aNb.sub.bX.sub.cZ.sub.dO.sub.n wherein X
is at least one element selected from the group consisting of Te
and Sb, Z is at least one element selected from the group
consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni,
Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth
elements and alkaline earth elements, a=0.1 to 1.0, b=0.01 to 1.0,
c=0.01 to 1.0, d=0 to 1.0, and n is determined by the oxidation
states of the other elements; and
(b) calcining the first catalyst partly or wholly in an atmosphere
that comprises water vapor to form a mixed metal oxide final
catalyst.
In another aspect, the disclosure provides a process for the
preparation of a catalyst for the (amm)oxidation of alkanes,
comprising the steps of:
(a) providing a precursor for a mixed metal oxide first catalyst,
the precursor having the empirical formula:
M.sub.eMoV.sub.aNb.sub.bX.sub.cZ.sub.dO.sub.n wherein M.sub.e is at
least one chemical modifying agent selected from the group
consisting of a reducing agent, an oxidizing agent, and an alcohol,
X is at least one element selected from the group consisting of Te
and Sb, Z is at least one element selected from the group
consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni,
Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth
elements and alkaline earth elements, a=0.1 to 1.0, b=0.01 to 1.0,
c=0.01 to 1.0, d=0 to 1.0, n is determined by the oxidation states
of the other elements, and e is 0 or a positive number; and
(b) calcining said precursor to form said mixed metal oxide first
catalyst; and
(c) after said calcining, re-calcining the first catalyst partly or
wholly in an atmosphere that comprises water vapor to form a mixed
metal oxide final catalyst.
Surprisingly, the process of the present disclosure is a simple
process to improve the yield to an (amm)oxidation catalyst, i.e.
steam re-calcination surprisingly improves the yield of the
starting, or first, catalyst that is subjected to the process.
In one embodiment, the process of the present disclosure uses fewer
steps to prepare a catalyst that provides similar, or improved,
yield compared to catalysts prepared by more complex prior art
methods. In addition, it minimizes the handling, e.g. transferring
of the catalyst intermediate from one step to another step, of the
catalyst as the precursor calcination and steam re-calcination can
be conducted sequentially without removing the catalyst from the
calcining furnace between the two calcination steps.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an X-ray diffraction pattern of a first catalyst prepared
in Comparative Experiment 2.
FIG. 2 is an X-ray diffraction pattern of a final catalyst prepared
in Example 3.
DETAILED DESCRIPTION OF THE INVENTION
In one embodiment, the process disclosed herein involves calcining
a catalyst in the presence of water vapor. In one embodiment, the
catalyst preparation process comprises preparing a first mixed
metal oxide catalyst by calcining a precursor to produce a first
catalyst, followed by calcining the first catalyst in the presence
of water vapor to produce a final catalyst.
As used herein, "a," "an," "the," "at least one," and "one or more"
are used interchangeably. The terms "comprises," "includes," and
variations thereof do not have a limiting meaning where these terms
appear in the description and claims. Thus, for example, an aqueous
composition that includes particles of "a" hydrophobic polymer can
be interpreted to mean that the composition includes particles of
"one or more" hydrophobic polymers.
Also herein, the recitations of numerical ranges and/or numerical
values, including such recitations in the claims, can be read to
include the term "about." In such instances the term "about" refers
to numerical ranges and/or numerical values that are substantially
the same as those recited herein.
Also herein, the recitations of numerical ranges by endpoints
include all numbers subsumed in that range (e.g., 1 to 5 includes
1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). For the purposes of the
invention, it is to be understood, consistent with what one of
ordinary skill in the art would understand, that a numerical range
is intended to include and support all possible subranges that are
included in that range. For example, the range from 1 to 100 is
intended to convey from 1.01 to 100, from 1 to 99.99, from 1.01 to
99.99, from 40 to 60, from 1 to 55, etc.
As used herein, the term "(meth)acrylic" refers to acrylic or
methacrylic.
The catalyst that is subjected to calcination in the presence of
water vapor is referred to herein as the "first catalyst."
For the process of this invention, the first catalyst can be any
MMO catalyst capable of (amm)oxidizing alkanes to unsaturated
carboxylic acids or nitriles. In one embodiment, the first catalyst
has the empirical formula: MoV.sub.aNb.sub.bX.sub.cZ.sub.dO.sub.n
wherein X is at least one element selected from the group
consisting of Te and Sb, Z is at least one element selected from
the group consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co,
Rh, Ni, Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare
earth elements and alkaline earth elements, a=0.1 to 1.0, b=0.01 to
1.0, c=0.01 to 1.0, d=0 to 1.0, and n is determined by the
oxidation states of the other elements. In one embodiment, the
first catalyst is a promoted MMO in which Z is present, preferably
with a value of d of from 0.001 to 0.1. Examples of promoted MMO
catalysts are described, e.g., in U.S. Pat. Nos. 7,304,014,
6,825,380; 6,790,988; 6,700,015; 6,504,053 and 6,407,280. In
another embodiment, Z is absent (d=0).
Preferably, when a=0.1 to 0.5, b=0.1 to 0.5, c=0.05 to 0.5, and
d=0.005 to 0.5. More preferably, when a=0.2 to 0.4, b=0.15 to 0.45,
c=0.05 to 0.45, and d=0.005 to 0.1. However, in an alternative
embodiment, when a=0.2 to 0.3, b=0.1 to 0.3, c=0.15 to 0.3 and
d=0.005 to 0.02. The value of n, i.e. the amount of oxygen present,
is dependent on the oxidation state of the other elements in the
catalyst. However, n is advantageously in the range of from 3.5 to
4.7. Preferably, X is Te. Preferably, Z is Pd.
The X-ray diffraction pattern of a suitable first catalyst is shown
in FIG. 1. The relative peak intensities may vary slightly due to
small variations in catalyst composition or calcination conditions,
however, it is preferred for optimal catalytic performance in
selective propane oxidation to acrylic acid that the orthorhombic
phase should be the major crystal phase with main peaks with
2.theta. at 6.7.degree., 7.8.degree., 22.1.degree., and
27.2.degree..
The MMO first catalyst can be formed from a precursor according to
methods well known to those skilled in the art. For example, the
precursor can be prepared using an aqueous slurry or solution
comprising solutions containing the MMO component metals. Water can
be removed by any suitable method known in the art to form a
catalyst precursor. Such methods include, without limitation,
vacuum drying, freeze drying, spray drying, rotary evaporation and
air drying. Conditions for drying MMO catalyst precursors are known
and may be found in the patents cited hereinabove.
The precursor for the mixed metal oxide first catalyst
advantageously can have the empirical formula:
M.sub.eMoV.sub.aNb.sub.bX.sub.cZ.sub.dO.sub.n wherein M.sub.e is at
least one chemical modifying agent selected from the group
consisting of a reducing agent, an oxidizing agent, and an alcohol,
X is at least one element selected from the group consisting of Te
and Sb, Z is at least one element selected from the group
consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni,
Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth
elements and alkaline earth elements, a=0.1 to 1.0, b=0.01 to 1.0,
c=0.01 to 1.0, d=0 to 1.0, n is determined by the oxidation states
of the other elements, and e is 0 or a positive number.
Once obtained, the catalyst precursor can be calcined to form the
MMO first catalyst. Calcination of the catalyst precursor is
referred to hereinafter as the "precursor calcination." The
precursor calcination may be conducted partially in an
oxygen-containing atmosphere or in the substantial absence of
oxygen, e.g., in an inert atmosphere or in vacuo. The inert
atmosphere may be any material that is substantially inert, i.e.,
does not react or interact with, the catalyst precursor. Suitable
examples include, without limitation, nitrogen, argon, xenon,
helium or mixtures thereof. Preferably, the inert atmosphere is
argon or nitrogen. The inert atmosphere may flow over the surface
of the catalyst precursor or may not flow thereover (a static
environment). When the inert atmosphere does flow over the surface
of the catalyst precursor, the flow rate can vary over a wide
range, e.g., at a gas hourly space velocity of from 1 to 500
hr..sup.-1.
The precursor calcination is advantageously performed at a
temperature of from 350.degree. C. to 850.degree. C. Preferably,
the precursor calcination is performed at least at 400.degree. C.,
more preferably at least at 500.degree. C. Preferably, the maximum
precursor calcination temperature is 750.degree. C., more
preferably 700.degree. C. The precursor calcination is performed
for an amount of time suitable to form the aforementioned first
catalyst. Advantageously, the precursor calcination is performed
for from 0.1 to 72 hours, preferably from 0.5 to 25 hours, more
preferably for from 0.5 to 6 hours, to obtain the desired first
catalyst.
In a preferred mode of operation, the catalyst precursor is
calcined in two stages, i.e. the precursor calcination has two
stages. In the first stage of the precursor calcination, the
catalyst precursor advantageously is calcined in an inert or
oxidizing environment (e.g. air) at a temperature of from
200.degree. C. to 330.degree. C., preferably from 275.degree. C. to
325.degree. C. for from 15 minutes to 40 hours, preferably for from
0.5 to 6 hours. In the second stage of the precursor calcination,
the material from the first stage advantageously is calcined in a
non-oxidizing environment (e.g., an inert atmosphere) at a
temperature of from 500.degree. C. to 750.degree. C., preferably
for from 550.degree. C. to 650.degree. C., for 0.2 to 25 hours,
preferably for from 1 to 3 hours.
In a particularly preferred mode of operation, the catalyst
precursor in the first stage of the precursor calcination is placed
in the desired oxidizing atmosphere at room temperature and is then
heated to the first stage calcination temperature and held there
for the desired first stage calcination time. The atmosphere is
then replaced with the desired non-oxidizing atmosphere for the
second stage of the precursor calcination, and the temperature is
then raised to the desired second stage calcination temperature and
held there for the desired second stage calcination time.
Once obtained, the first catalyst can be calcined to form the final
catalyst. The calcination of the first catalyst is conducted in the
presence of water vapor, such as steam. This calcination is
referred to hereinafter as the steam re-calcination.
Advantageously, the steam re-calcination produces a catalyst that
may be employed to produce an (amm)oxidation process product, such
as acrylic acid, in a higher yield, measured at 85% oxygen
conversion, than the first catalyst from which it is prepared.
In one embodiment, the average amount of water vapor present in the
steam re-calcination is from about 0.01 to about 100, preferably
from 0.5 to 10, more preferably from 1 to about 3.5, volume
percent, based on the total volume of gas in the calcining vessel.
The steam re-calcination may be conducted partially in an
oxygen-containing atmosphere or in the substantial absence of
oxygen, e.g., in an inert atmosphere or in vacuo. The inert
atmosphere may be any material that is substantially inert, i.e.,
does not react or interact with, the catalyst precursor. Suitable
examples include, without limitation, nitrogen, argon, xenon,
helium or mixtures thereof. Preferably, the inert atmosphere is
argon or nitrogen. The inert atmosphere may flow over the surface
of the catalyst precursor or may not flow thereover (a static
environment). When the inert atmosphere does flow over the surface
of the catalyst precursor, the flow rate can vary over a wide
range, e.g., at a space velocity of from 1 to 500 hr..sup.-1.
The steam re-calcination advantageously is performed at a
temperature of from 100.degree. C. to 650.degree. C. Preferably,
the steam re-calcination is performed at a temperature of at least
300.degree. C. Preferably, the maximum steam re-calcination
temperature is 650.degree. C., more preferably 550.degree. C. The
steam re-calcination suitably is performed for an amount of time
suitable to form the final catalyst. Advantageously, the steam
re-calcination is performed for from 0.1 to 72 hours, preferably
from 0.5 to 25 hours, more preferably for from 1 to 10 hours, to
obtain the desired calcined final modified mixed metal oxide.
Water vapor can be introduced at any time in the steam
re-calcination. When two stages are employed in the steam
re-calcination, water vapor can be introduced in either the first
stage, second stage, or both stages. The water vapor can be present
through a whole stage of the calcination, or only part of a stage.
In a particularly preferred mode of the operation, the water vapor
is introduced at least in the first stage of the steam
re-calcination.
Water vapor can be introduced alone or together with a carrier gas.
In one mode of operation, the water vapor is introduced by passing
a carrier gas through a water saturator that is maintained at room
temperature. In another mode of operation, the water can be
injected in liquid form and then immediately vaporized into vapor
prior to contacting the catalyst. Other modes of operation are also
possible, such as generating water vapor and introducing it
directly into the calcination vessel.
The water vapor can flow through the catalyst particles, or contact
the catalyst particles in static mode, as long as substantial
contact of water vapor with the catalyst particles occurs.
The purity of the water vapor is not particularly critical,
although it is preferred that the purity does not have a
substantial negative impact on catalyst performance. In one
preferred mode of operation, deionized water is employed.
In a preferred mode of operation, the first catalyst advantageously
is calcined in two stages in the steam re-calcination step. In the
first stage, the first catalyst advantageously is calcined in an
inert or oxidizing environment (e.g. air) at a temperature of from
100.degree. C. to 400.degree. C., preferably from 275.degree. C. to
325.degree. C. for from 15 minutes to 40 hours, preferably for from
0.5 to 6 hours. In the second stage, the material from the first
stage advantageously is calcined in a non-oxidizing environment
(e.g., an inert atmosphere) at a temperature of from 400.degree. C.
to 650.degree. C., preferably for from 450.degree. C. to
550.degree. C., for 0.5 to 25 hours, preferably for from 1 to 3
hours.
In a particularly preferred mode of operation, the first catalyst
in the first stage is placed in the desired oxidizing atmosphere at
room temperature and then raised to the first stage calcination
temperature and held there for the desired first stage calcination
time. The atmosphere is then replaced with the desired
non-oxidizing atmosphere for the second stage calcination, the
temperature is raised to the desired second stage calcination
temperature and held there for the desired second stage calcination
time.
Although any type of calcination vessel and heating mechanism,
e.g., a furnace, may be utilized during calcination, it is
preferred to conduct calcination under a flow of the designated
gaseous atmosphere. Therefore, it is advantageous to conduct
calcination in a bed with continuous flow of the desired gas(es)
through the bed of solid catalyst or catalyst precursor
particles.
The first and/or final metal oxide catalyst may be ground at any
point in the process, following or prior to any of the treatment
steps. In a preferred embodiment, only the final catalyst is
ground. Preferably, the surface area after grinding is from 5-30
m.sup.2/g, and more preferably is from 10-18 m.sup.2/g. Examples of
suitable types of grinding apparatus include, e.g., a freezer/mill,
ball mill, mortar and pestle, and jet mill.
The final catalyst can be used as an (amm)oxidation catalyst. For
example, it can be employed to oxidize propane to acrylic acid.
Similarly, methacrylic acid can be produced by isobutane oxidation.
In addition to (meth)acrylic acid, (meth)acrylonitrile can be
produced by oxidizing propane or isobutane in the presence of air
and ammonia. Although the catalyst shows big advantages when used
in alkane (amm)oxidation, it can be used in (amm)oxidation of
olefins or a mixture of olefins and alkanes.
SPECIFIC EMBODIMENTS OF THE INVENTION
The following examples are given to illustrate the invention and
should not be construed as limiting its scope.
EXAMPLES
Comparative Experiment 1--Synthesis of MMO Precursor
A mixed oxide first catalyst of nominal composition
Mo.sub.1.0V.sub.0.285Te.sub.0.21Nb.sub.0.164Pd.sub.0.01O.sub.n is
produced from a MMO precursor prepared in the following manner:
(1) An aqueous solution containing Mo(VI), V(V), and Te(IV) salts
is formed by dissolving ammonium heptamolybdate tetrahydrate (35.7
g, from Fisher Scientific), ammonium metavanadate (6.7 g, from
Sigma Aldrich) and telluric acid (9.7 g, from Sigma Aldrich)
sequentially in 200 ml of deionized (D.I.) water that is pre-heated
to 70.degree. C. The mixed salt solution is stirred for 20 minutes
at 70.degree. C. to ensure that a clear solution is formed. Then, 5
ml of concentrated nitric acid (70 wt. % in water, from Sigma
Aldrich) is added to the solution under stirring.
(2) Separately, an aqueous solution containing ammonium niobium
oxalate (15.7 g, from H.C. Starck, Goslar, Germany) and oxalic acid
dihydrate (3.9 g, from Sigma Aldrich) in 180 cc D.I. water is
prepared at room temperature.
(3) The Mo/V/Te solution is held under stirring with heating. Then,
the heating is stopped and the Nb-containing solution is added to
the Mo/V/Te. Gelation occurs immediately after the two solutions
are mixed. The mixture is stirred for 5 minutes and becomes a
slurry before being transferred to a round-bottom rotary flask.
(4) The water in the slurry is removed via a rotary evaporator at
50.degree. C. under a vacuum of 10-50 mmHg (1.33-6.67 kPa). The
solid material is further dried in a vacuum oven overnight at room
temperature. This dried material is designed as the "MMO
precursor."
Comparative Experiment 2--Calcination of MMO Precursor
The MMO precursor (15-20 grams) prepared in Comparative Experiment
1 is placed in the middle of a quartz tube with quartz wool stuffed
into the tube at both ends of the solid bed of precursor. The
precursor is calcined by heating the tube furnace from room
temperature to 275.degree. C. at 10.degree. C./min in flowing air
(80-100 standard cubic centimeters per minute, hereinafter SCCM)
and holding the temperature at 275.degree. C. for one hour. The
flowing gas is then switched to inert gas such as argon or nitrogen
(80-100 SCCM). The furnace temperature is raised to 615.degree. C.
from 275.degree. C. at 2.degree. C./min and held at 615.degree. C.
in an inert gas atmosphere for two hours. This results in a first
catalyst of nominal composition
Mo.sub.1.0V.sub.0.285Te.sub.0.21Nb.sub.0.164Pd.sub.0.01O.sub.n.
The MMO solid first catalyst has an X-ray diffraction pattern as
shown in FIG. 1.
Comparative Experiment 3--Grinding of MMO First Catalyst
The MMO first catalyst of Comparative Experiment 2 is broken into
smaller pieces and passed through 10 mesh sieves. The sieved
particles are ground with a 6850 Freezer/Mill (from SPEX Certiprep,
located at Metuchen, N.J., USA). Grinding time is adjusted to get a
final ground material having a surface area in the range of 12-14
m.sup.2/g. This results in a ground first catalyst of nominal
composition
Mo.sub.1.0V.sub.0.285Te.sub.0.21Nb.sub.0.164Pd.sub.0.01O.sub.n.
Example 1--Preparation of MMO Final Catalyst with Steam
Introduction at 100.degree. C.
A MMO first catalyst with the composition
Mo.sub.1.0V.sub.0.285Te.sub.0.21Nb.sub.0.164Pd.sub.0.01O.sub.n is
prepared by following the procedure of Comparative Experiments 1
and 2. The first catalyst is re-calcined in the presence of steam
in the same tube furnace. This post calcination is hereinafter
designated a "steam re-calcination" step. The carrier gas air flows
through a glass water saturator held at room temperature, and then
passes through the catalyst bed. The water concentration in the
flowing air is 3.0 volume % based on the equilibrium water partial
pressure at room temperature (22-25.degree. C.). In this example,
the temperature of the furnace is increased from ambient
temperature to 100.degree. C. with a ramp rate of 5.degree. C./min,
and then is held for three hours at 100.degree. C. Then, the
flowing air bypasses the water saturator and continues flowing
through the catalyst bed, i.e. water vapor is no longer introduced,
while the bed temperature is raised at 5.degree. C./min to
300.degree. C. and is held there for three hours. After calcination
at 300.degree. C., the flowing gas is switched to argon and the bed
temperature is raised at 2.degree. C./min to 500.degree. C. and is
held there for two hours. The re-calcined material is ground to
fine powder following the procedure of Comparative Experiment
3.
Example 2--Preparation of MMO Final Catalyst with Steam
Introduction at 200.degree. C.
The procedure of Example 1 is repeated except that the steam
introduction temperature is 200.degree. C.
Example 3--Preparation of MMO Final Catalyst with Steam
Introduction at 300.degree. C.
The procedure of Example 1 is repeated except that the steam
introduction temperature is 300.degree. C. The MMO final catalyst
after the re-calcination in steam and air/argon has an X-ray
diffraction pattern as shown in FIG. 2.
Example 4--Preparation of MMO Final Catalyst with Re-Calcination
Under Steam at 300.degree. C. and no Air Re-Calcination at
300.degree. C.
The procedure of Example 1 is repeated except that (1) the steam is
introduced while the catalyst is heated to 300.degree. C. and is
held at 300.degree. C. for 3 hours, and (2) the air re-calcination
at 300.degree. C. following steam re-calcination is skipped.
Example 5--Preparation of MMO Final Catalyst with Re-Calcination
Under Steam at 300.degree. C. and 500.degree. C.
The procedure of Example 1 is repeated except that: (1) the steam
is introduced while the catalyst is heated to 300.degree. C. and is
held at 300.degree. C. for 3 hours, (2) the air re-calcination at
300.degree. C. following steam re-calcination is skipped, and (3)
steam is introduced continuously during the re-calcination to and
at 500.degree. C. in argon.
Comparative Experiment 4--Preparation of MMO Comparative Catalyst
with Re-Calcination without Steam
The procedure of Example 1 is repeated except that the steam
introduction to and at 100.degree. C. is skipped. The catalyst is
re-calcined in flowing air first at 300.degree. C. for 3 hours, and
then in flowing argon at 500.degree. C. for 2 hours.
Comparative Experiment 5--Preparation of MMO Comparative Catalyst
with Steam Introduction During MMO Precursor Calcination at
275.degree. C.
A MMO catalyst with the composition
Mo.sub.1.0V.sub.0.285Te.sub.0.21Nb.sub.0.164Pd.sub.0.01O.sub.n is
prepared following the procedure of Comparative Experiment 3 except
that steam is introduced during precursor calcination to and at
275.degree. C. The steam is introduced by passing the air through a
water saturator held at room temperature prior to sending the air
through the catalyst bed.
Comparative Experiment 6--Preparation of MMO Comparative Catalyst
with Steam Introduction During the Entire MMO Precursor Calcination
Step
A MMO catalyst with the composition
Mo.sub.1.0V.sub.0.285Te.sub.0.21Nb.sub.0.164Pd.sub.0.01O.sub.n is
prepared following the procedure of Comparative Experiment 3 except
that steam is introduced during the entire precursor calcination
process. The steam is introduced by passing the carrier gas through
a water saturator held at room temperature prior to sending the
carrier gas through the catalyst bed.
Comparative Experiment 7--Preparation of MMO Comparative Catalyst
with Subsequent Water Wetting at Room Temperature and
Re-Calcination/Grinding
20 grams of the first catalyst of Comparative Experiment 2 is
stirred in D.I. water (50 g) for twenty minutes, and dried via a
rotary evaporator at 50.degree. C. under a vacuum of 10-50 mmHg
(1.33-6.67 kPa). The dried material is then calcined in flowing air
at 300.degree. C. for 3 hours, and then calcined in flowing argon
at 500.degree. C. for 2 hours. This re-calcined material is then
ground following the procedure of Comparative Experiment 3.
Comparative Experiment 8--Preparation of MMO Comparative Catalyst
with Contacting Water Under Reflux Condition and
Re-Calcination/Grinding Steps
20 grams of this the calcined first catalyst of Comparative
Experiment 2 is heated in D.I. water (50 g) under reflux for 5
hours. The resulting material is collected by vacuum filtration and
dried in a vacuum oven at room temperature overnight. The dried
material is first calcined in flowing air at 300.degree. C. for 3
hours, and then calcined in flowing argon at 500.degree. C. for 2
hours. This re-calcined material is ground following the procedure
of Comparative Experiment 3.
Comparative Experiment 9--Preparation of MMO Comparative Catalyst
with Extraction Step
10 grams of the first catalyst of Comparative Experiment 3 is added
to 100 grams of an aqueous solution containing 2 wt. % of oxalic
acid. The mixture is stirred under reflux for 5 hours. The material
is collected by vacuum filtration and dried in a vacuum oven at
room temperature overnight.
Comparative Experiment 10--Preparation of MMO Comparative Catalyst
with Multiple Synthesis Steps Including Impregnation and
Extraction
A MMO catalyst with the composition
Mo.sub.1.0V.sub.0.285Te.sub.0.21Nb.sub.0.164Pd.sub.0.01O.sub.n is
synthesized following the sequence of multiple steps listed
below:
(a) a MMO precursor is synthesized following the procedure of
Comparative Experiment 1;
(b) the MMO precursor is calcined as in Comparative Experiment
2;
(c) the calcined MMO is impregnated with tellurium and niobium. 60
grams of the calcined MMO catalyst of step (b) is added to a
solution containing telluric acid (0.54 g, from Sigma Aldrich),
ammonium niobium oxalate (0.75 g, from H.C. Starck located at
Goslar, Germany), and 100 grams of D.I. water. The resulting
mixture is stirred for 20 minutes at room temperature and dried via
a rotary evaporator at 50.degree. C. under a vacuum of 10-50 mmHg
(1.33-6.67 kPa). This step is designated as the "impregnation"
step.
(d) 10 grams of the dried material impregnated with tellurium and
niobium is re-calcined first in flowing air at 300.degree. C. for 3
hours, and then in flowing argon at 500.degree. C. for 2 hours;
(e) The re-calcined material is ground following procedure in
Comparative Experiment 3;
(f) 10 grams of the ground powder is added to 100 grams of an
aqueous solution containing 2 wt. % of oxalic acid under stirring.
The mixture is heated to reflux and is maintained at reflux for 3
hours;
(g) The resulting solid material is collected by vacuum filtration
and is dried in a vacuum oven at room temperature overnight.
Evaluation of MMO Catalysts for Propane Oxidation to Acrylic
Acid
The catalysts obtained from the above examples and comparative
experiments are each pressed and sieved to 14-20 mesh granules. 4
grams of granules of each catalyst are packed into a stainless
steel plug flow tubular reactor (inside diameter: 1.1 cm) with
silicon carbide inert particles loaded above and below the catalyst
charge. The reactor tube is installed in an electrically heated
clamshell furnace. The catalyst bed is preheated to 180.degree. C.
in a 50 SCCM N.sub.2 flow, and then the gas feed is switched to a
gas mixture feed containing 7 vol. % propane, 70 vol. % air, and 23
vol. % steam. Steam is provided by injecting D.I. water via a
syringe pump into a pre-heating zone set at 180.degree. C. The flow
rate of the feed is controlled at 80 SCCM with a residence time of
3 seconds. The catalyst bed temperature is adjusted to obtain the
desired conversion of oxygen.
The effluent of the reactor is condensed by a cold trap submerged
in an ice/water bath to collect condensable products. Reaction
product vapor exiting the cold trap is analyzed by a gas
chromatograph equipped with a thermal conductivity detector and
molecular/silical gel columns. The condensed liquid products are
analyzed by a gas chromatograph fitted with a flame ionization
detector and an Alltech ECONO-CAP EC-1000 capillary column (30
m.times.0.53 mm ID.times.1.2 .mu.m).
The conversion of O.sub.2 is calculated by difference as follows,
where n.sub.i denotes the molar flow rate of species i:
.times..times..times..times..times..times..times..times..times.
##EQU00001## The yield of acrylic acid (AA) is calculated as
follows:
.times..times..times..times..times. ##EQU00002##
Acrylic acid yields of the different catalysts are compared at a
constant O.sub.2 conversion of 85%. In the manufacture of acrylic
acid from propane, it is desirable to maximize reactor
productivity, to maximize selectivity to product, to avoid the
creation of a flammable mixture of gases, and to maximize catalyst
lifetime. To maximize productivity, concentrations of propane and
oxygen in the reactor feed are increased. To maximize product
yield, the concentration of water (steam) in the feed is increased.
To avoid the creation of a flammable mixture of gases, the ratio of
fuel to oxygen in the feed is controlled. To maximize catalyst
lifetime, a minimal amount of oxygen is maintained in the reactor
effluent. The net effect of the feed constraints is to require a
feed mixture in which the ratio of propane to oxygen is nearly
stoichiometric. That is, for the conversion of propane to acrylic
acid, the ratio of propane to oxygen is about 1:2.1. Since the
formation of waste products (carbon oxides, acetic acid) requires a
greater amount of oxygen, and their formation is unavoidable,
oxygen becomes the limiting reagent in the reaction. For this
reason, it is preferred to measure catalyst performance as yield of
(or selectivity to) acrylic acid as a function of oxygen
conversion, rather than propane conversion.
All the catalyst samples are tested under the same conditions, such
as feed composition and residence time, as described above.
However, the reaction temperature is varied among different samples
due to activity differences in order to achieve 85% O.sub.2
conversion. The yields of acrylic acid at 85% O.sub.2 conversion
over different samples are listed in Table 1.
TABLE-US-00001 TABLE 1 Yield of acrylic acid over different MMO
catalyst samples synthesized under different conditions/steps Steps
Involved for MMO Catalyst Synthesis Starting from MMO Precursor
Precursor Impreg- Mixing Re-calcination (.degree. C.) Extraction
& AA Calcination nation & w/water & with without
Filtration Yield Example (.degree. C.) Drying Drying** steam steam
& Drying Grinding (%) CE 3* X X 54.0 1 X X X X 56.9 (100)
(300/500) 2 X X X X 57.1 (200) (300/500) 3 X X X X 57.6 (300)
(300/500) 4 X X X X 57.5 (300) (500) 5 X X X 56.3 (300/500) CE 4* X
X X 54.5 (300/500) CE 5* X X 54.6 (w/steam at 275) CE 6* X X 54.0
(w/steam at 275/615) CE 7* X X X X 56.0 (mixing at (300/500) r.t.)
CE 8* X X X X 53.0 (refluxing (300/500) in water) CE 9* X X X 56.0
CE 10* X X X X X 59.0 *CE = Comparative Experiment - not an
embodiment of the invention. **the catalyst particles are recovered
by filtration after refluxing in water.
These results clearly demonstrate the benefits of steam
re-calcination of the first MMO catalyst to prepare the final
catalyst. As shown in Table 1, MMO final catalysts prepared by
simple steam re-calcination of a MMO first catalyst show an AA
yield of around 57%, which is about 3-4% higher than the yield
achieved using the first MMO catalyst of C.E. 3, which is prepared
without further steam re-calcination.
The re-calcined catalysts also unexpectedly show improved AA yield
compared to the comparative catalysts prepared by more complex
methods of C.E.s 4-9.
While the MMO catalyst prepared via multiple steps as shown in
Comparative Experiment 10 gives an AA yield of 59%, its preparation
involves at least eight steps (both filtration and drying are
counted as single step) with frequent transferring of the catalyst
intermediate between different steps. These multiple synthesis
steps not only increase the catalyst cost significantly, but
increase the risk of variation of the final catalyst
performance.
* * * * *
References